With injuries maybe forcing changes, just dont think Costa warrants a starting place. Present form less than mediocre, and considering all the hype / transfer fee paid, generally has been a huge flop. Lightweight, ponderous, prone to foul, spends to much time on his backside.
As been said, time for Gelhardt, Greenwood, Summerville plus others to be given their chance. If we dont play them against the big boys, its hard to gauge if they can make the step up or not.
Other clubs, Liverpool / Arsenal etc, seem more open to give their U23s a go, even with 1st team squads larger than ours.